1) Field of the Invention
The present invention relates to a system and a method for controlling a spark-ignition internal combustion engine to have lesser combustion fluctuations.
2) Description of the Related Art
A conventional technology for controlling spark-ignition internal combustion engines is disclosed in Japanese Patent Application Laid Open No. 2001-20837. The disclosed spark-ignition internal combustion engine includes fuel injection valves arranged in both a cylinder and an intake port. Such a structure is adopted to maintain both stratified charge combustion and homogeneous combustion in their satisfactory states. The disclosed control system changes the proportion between direct injection and intake-port injection based on an operating state of the internal combustion engine.
However, in the conventional technology, amount of injected fuel may fluctuate or combustion fluctuations may occur. The amount of fuel can fluctuate due to changes in the injection characteristics. The injection characteristics change due to deposition of material in a direct injection valve or changes in amount of heat received from combustion gas. On the other hand, the combustion fluctuations may occur because combustion deteriorates when mixture is made insufficiently homogeneous caused by a shortage of time to vaporize the fuel injected.